Roots supercharger with a shunt pulsation trap

ABSTRACT

A shunt pulsation trap for a Roots supercharger reduces pulsation, NVH and improves efficiency without significantly increasing overall size of the supercharger. Generally, a Roots supercharger with the shunt pulsation trap has a pair of interconnected and synchronized parallel multi-helical-lobe rotors housed in a transfer chamber with the same number of lobes for propelling flow from a suction port to a discharge port of the transfer chamber without internal compression. The shunt pulsation trap comprises an inner casing as an integral part of the transfer chamber, and an outer casing oversized surrounding the inner casing, therein housed various pulsation dampening means or pulsation energy recovery means or pulsation containment means, at least one injection port (trap inlet) branching off from the transfer chamber into the pulsation trap chamber and a feedback region (trap outlet) communicating with the supercharger outlet pressure.

CLAIM OF PRIORITY

This application claims priority to Provisional U.S. patent application entitled ROOTS SUPERCHARGER WITH A SHUNT PULSATION TRAP, filed Jul. 20, 2010, having application No. 61/366,140, the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of rotary blowers or compressors used in automotive supercharging applications, and more particularly relates to a double rotor helical shaped multi-lobe type commonly known as Roots blowers or superchargers (other often used names are rotary PDs, rotary lobe or rotary piston types), and more specifically relates to a shunt pulsation trap for reducing pulsations and induced vibration, noise and harshness (NVH) from such superchargers for internal combustion engines.

2. Description of the Prior Art

Ever since German engineer Gottlieb Daimler filed the first patent in late 19^(th) century, the Roots supercharger has been most widely used in supercharging automotive engines until turbocharging took its place. However, they are still popular for all kinds of 2-stroke and 4-stroke cycle engines either gasoline or diesel.

It has long been known that Roots blower or supercharger possesses a unique capability for generating adequate discharge pressures over a wide speed range. This unique variable pressure adaptability is attributed better to a wave Roots compression theory postulated by this author. Inside a Roots blower or a supercharger, air is not compressed “by a rapid backflow without internal compression” as the conventional Roots principle has been believed, but instead it is compressed by a series of pressure waves or shock waves generated by a sudden opening of lobes to the supercharger discharge pressure. The wave theory is based on a well studied physical phenomenon as occurs in a shock tube (invented in 1899) where a diaphragm separating a region of high-pressure gas from a region of low-pressure gas inside a closed tube. As shown in FIG. 1 a-1 b, when the diaphragm is suddenly broke open, a series of expansion waves is generated propagating from the low-pressure to the high-pressure region at a speed of sound, and simultaneously a series of pressure waves which quickly coalesces into a shockwave is propagating from the high-pressure to the low-pressure region at a speed faster than the speed of sound. An interface, also referred to as the contact surface that separates low and high pressure gases, follows at a lower velocity after the shock wave. Further compression is achieved by the reflected shock wave at the end wall of the low pressure region to the level very close to the final equilibrium pressure.

To understand the Roots compression principle in light of the shock tube theory, let's review a cycle of a classical Roots supercharger as illustrated from FIGS. 2 a to 2 e by following one flow cell in a typical three-lobe configuration. In FIGS. 2 a, low pressure air first enters the spaces between lobes of a pair of rotors axially as they are open to inlet during their outward rotation from inlet to outlet. At lobe position shown in FIG. 2 b, the air becomes trapped between two lobes and supercharger inner casing as it is transported from inlet to outlet. Then the trapped air is suddenly opened to higher pressure of the outlet as shown in FIG. 2 c.

According to the conventional theory, a backflow would rush in compressing the air inside the cell at this point as shown in FIG. 2 c. Since it is almost instantaneous and there is no volume change taking place, the compression is regarded as an iso-choric process (constant volume). After the compression, the rotors continue to move against this full pressure difference until lobes from two rotors meet again, meshing out the compressed air to outlet chamber and return to inlet suction position to start the next cycle, as shown in FIG. 2 d.

However, according to the shock tube theory, the lobe opening phase as shown in FIG. 2 c resembling the diaphragm bursting of a shock tube as shown in FIG. 1 b would generate a series of compression waves or a shock wave. The wave front sweeps through the low pressure air and compresses it at the same time at a speed faster than the speed of sound. This results in an almost instantaneous wave compression well before the induced flow interface (backflow as in conventional theory) could arrive because wave travels much faster than the fluid, as illustrated by the wave propagation in FIG. 2 e. In this view, the pressure waves or shock waves are the primary driver for the Roots compression while the backflow is simply an induced flow behind the shockwave after compression takes place.

From the above Roots cycle analysis, it should be noted that energy transfers directly between two fluids by waves without using mechanical components like pistons or vaned impellers. Their major benefits are their potentials to generate large pressure changes in short time or small distance in an efficiency equivalent to those of dry screw compressors of the present times. Moreover, there is no over-compression or under-compression as in the case of conventional positive displacement compressors with a pre-determined pressure ratio, a unique ability to adapt to varying pressure demands. This makes Roots supercharger ideal for variable pressure applications such as in automotive supercharging at different speeds or different pressure boosting levels while maintaining a good efficiency throughout the process. Since the compression is achieved through faster moving waves or shock waves without hardware or the associated inertial, Roots supercharger can be build very small in size and simple in structure without complicated geometry or rotor contours.

Despite the above mentioned generally attractive features for Roots potentials, several challenges have impeded their extensive commercial applications. Among them, the number one issue is pulsation control. According to the wave Roots theory, when pressure waves or shockwaves are generated on low pressure side compressing the air inside the lobe cell, a series of expansions waves are generated simultaneously on high pressure side. Those large amplitude expansion waves combined with the reflected pressure wave or shockwaves from the lobe cells, if not blocked or treated, could travel downstream, creating huge pressure and flow pulsations and induced vibrations that could destroy downstream components, or generate noises as high as 170 dB for high pressure applications. Therefore, a large sized pulsation dampener, either in the form of a plenum or a reactive type, is usually required at the discharge stream of a Roots supercharger to dampen the air borne pulsations. It is generally very effective in pulsation control but requires large size to be effective, not suitable for mobile applications such as automobiles and trucks. At the same time, discharge dampeners used today could create high pressure losses that contribute to poor supercharger efficiency. For this reason, Roots superchargers are often cited with high pulsation, noise and low efficiencies, all of which prevent it from a wider use in spite of its unique merits due to wave compression.

Various attempts have been made to reduce Roots pulsations throughout years, but only limited successes have been achieved. The main reason for this failure is believed to be lacking an adequate Roots compression mechanism that could point to the root cause of pulsations. Traditionally, Roots compression has been regarded as a backflow mechanism instead of the wave mechanism as described above. Based on the conventional backflow principle which attributes sudden backflow as the cause of discharge pressure pulsations, most of the efforts have been focused on controlling this backflow. Among the methods, a flow feedback principle is most widely used, for example, as first disclosed in U.S. Pat. No. 4,215,977 to Weatherston, and later in U.S. Pat. No. 4,768,934 to Soeters (Eaton), U.S. Pat. No. 6,589,034 to Vorwerk (Ford) and U.S. Pat. No. 6,874,486 to Prior (GM). The idea is to feed back a portion of the outlet air through an injection port to the transfer chamber prior to discharging to the outlet, thereby gradually increasing the air pressure inside the cell and lengthening the pressure equalizing time, hence reducing discharge pressure spikes compared with a sudden opening at discharge. However, its effectiveness for pulsation attenuation is limited because it fails to recognize that the waves, not fluid flow, are the primary cause of the air-borne pulsations. In view of the wave compression theory, having a flow back earlier could reduce pulsations by elongating releasing time to discharge pressure. However, it failed to recognize hence attenuate the simultaneously generated expansion waves at the injection port that eventually travel down-stream unblocked, causing high pulsations. Moreover, the prior art failed to address the high flow losses associated with the high induced jet velocity through the injection port, resulting in a low supercharger efficiency that hampers it from being used more widely to more energy sensitive applications.

Since the amplitude of pressure pulsation in a supercharger is typically much higher than the upper limit of 140 dB set in classical acoustics, the small disturbance assumption or the resulting linear theory is inadequate to predict its behavior. Instead, the following rules can be used for large disturbances when the SPL is beyond 140 dB. These rules are based on the above discussed Shock Tube theory and can be used to judge the source of gas pulsation and quantitatively predict its amplitude and travel directions. In principle, these rules are applicable to the discharge process of any positive displacement fluid machines such as internal combustion engines, expanders and pneumatic motors, or compressors or pumps.

-   -   1. Rule I: For two closed compartments (either moving or         stationery) with different pressure levels p₃ and p₁ (FIG. 1 a),         there will be no pulsation generated if the two compartments are         kept isolated with each other     -   2. Rule II: If the divider between high pressure p₃ and low         pressure p₁ is suddenly removed (FIG. 1 b), it will trigger         pulsation generation at opening as a mixture of Pressure Waves         (PW) or a shock wave, Expansion Waves (EW) and an Induced Fluid         Flow (IFF) with magnitudes as follows:

PW=p ₂ −p ₁  (1)

EW=p ₃ −p ₂  (2)

IFF Velocity=(p ₂ −p ₁/(d ₁ ×W)  (3)

where d₁ is the density of low pressure region and W the speed of shock wave travelling into the low pressure region, and

p2=(p ₃ ×p ₁)^(1/2)  (4)

-   -   3. Rule III: the generated Pressure Waves (PW) or shock wave         travel at the shock wave speed W to low pressure region while         Expansion Waves (EW) move at the speed of sound in a direction         opposite to PW, while at the same time both waves induce an         unidirectional fluid flow (IFF) moving in the same direction as         the pressure waves (PW)         Pay attention to Rule #2 which gives the location of pulsation         source as place of sudden opening between p₃ and p₁. It also         indicates the sufficient conditions for gas pulsation generation         as existence of pressure difference and sudden opening. Because         all PD fluid machines convert energy between shaft and fluid by         dividing incoming continuous fluid stream into parcels of         compartment size for delivery to the discharge as indicated by         its corresponding cycle, there always exists a “sudden” opening         at discharge to return these discrete parcels of cavities back         to a continuous stream again. So the two sufficient conditions         are automatically satisfied at the moment of discharge opening         if there is a pressure difference existing between the cavity         and outlet it is opened to. The pulsation magnitude predicted by         Rule #2 can be very high if (p₃−p₁) is large enough for an         un-throttled (or infinitely fast) opening as in a shock tube.         However, most PD type fluid machines operate with finite         discharge opening speed which somehow throttles the induced         fluid flow to a maximum sonic velocity that takes places at a         pressure ratio of 1.89, say for a perfect gas with 1.4 specific         heat ratio. In addition, a hardware (like lobe or valve disk)         induced flow pulsation co-exists with pressure difference         induced pulsation, but its magnitude is typically much smaller         for most existing fluid machinery, and is roughly proportional         to its equivalent velocity pressure.

It should be pointed out the drastic magnitude and behavior difference between acoustic waves and pulsations discussed above. First of all, the linear acoustics is limited to pressure fluctuation level below 140 dB, equivalent to pressure level of 0.002 Bar or 0.03 psi. For fluid machinery, the measured pressure fluctuation or pulsation is often in the range of 0.3-30 psi (or even higher), equivalent to 160-200 dB. So pulsation pressures are much higher and well beyond the pressure range intended in classical acoustics. Physically, the acoustic waves are sound waves travelling at the speed of sound with no macro fluid movement with it while pulsations are a mixture of strong pressure and expansion waves that also induce an equally strong macro fluid flow travelling with speeds from a few centimeters per second up to 1.89 times of the speed of sound (Mach Number=1.89), for example. It is this large pressure forces and induced high velocity fluid flow that could directly damage a system and components on its travelling path, in addition to exciting vibrations and noises. With the above Pulsation Rules, it is hoped that more realistic pulsation prediction is made possible so that the true nature of pulsations can be realized, hence controlled.

Accordingly, it is always desirable to provide a new design and construction of a Roots supercharger that is capable of achieving high pulsation and NVH reduction at source and improving supercharger efficiency without externally connected silencers while being kept light in mass, compact in size and suitable for high efficiency, high pressure ratio applications at the same time.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a Roots supercharger with a shunt pulsation trap in parallel with the transfer chamber for trapping and attenuating pulsations at source.

It is a further object of the present invention to provide a Roots supercharger with a shunt pulsation trap as an integral part of the supercharger casing that does not need an externally connected pulsation dampener or silencer so that it remains light in weight and compact in size with less noise radiation surfaces.

It is a further object of the present invention to provide a Roots supercharger with a shunt pulsation trap that emits pulsation free gases downstream hence reduce fatigue failure of downstream components.

It is a further object of the present invention to provide a Roots supercharger with a shunt pulsation trap that is capable of achieving all the above objectives in a wide range of engine operating speeds and loads.

It is a further object of the present invention to provide a Roots supercharger with a shunt pulsation trap that is capable of achieving higher adiabatic efficiency in the range equivalent or close to conventional turbocharger or dry screw supercharger, say up to about 80%.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring particularly to the drawings for the purpose of illustration only and not limited for its alternative uses, there is illustrated:

FIGS. 1 a and 1 b show the pressure and wave distribution of a shock tube device before and after the diaphragm is broken;

FIG. 2 a to 2 d (PRIOR ART) show the Roots compression cycle of a conventional Roots supercharger;

FIG. 2 e shows the triggering mechanism for wave generation of a conventional Roots supercharger;

FIG. 3 a to 3 d show the wave Roots compression cycle of the present invention Roots supercharger with a shunt pulsation trap:

FIG. 3 e shows the triggering mechanism for wave generation of the present invention Roots supercharger with a shunt pulsation trap;

FIGS. 4 a and 4 b show a perspective and a cross-sectional side view of a preferred embodiment of the shunt pulsation trap also showing different shapes of injection port nozzle;

FIGS. 5 a and 5 b-c show a perspective and a cross-sectional side view of an alternative embodiment of the shunt pulsation trap with an additional wave reflector either after or before the feedback port

FIG. 6 is a cross-sectional view of different shapes of a wave reflector of the shunt pulsation trap;

FIG. 7 is a perspective view of another alternative embodiment of the shunt pulsation trap with resonators;

FIGS. 8 a, 8 b and 8 c show a perspective and cross-sectional side views of another alternative embodiment of the shunt pulsation trap with a diaphragm as a dampener and pump;

FIGS. 9 a and 9 b show a cross-sectional view of a rotary valve and a reed valve in open and close positions;

FIGS. 10 a, 10 b and 10 c show a perspective and cross-sectional side views of yet another alternative embodiment of the shunt pulsation trap with a piston as a dampener and pump;

FIGS. 11 a, 11 b and 11 c show a perspective and cross-sectional side views of yet another alternative embodiment of the shunt pulsation trap with a diaphragm used as a dampener pump to drive an external load;

FIGS. 12 a and 12 b show a perspective and cross-sectional side views of yet another alternative embodiment of the shunt pulsation trap with a valve at trap outlet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Although specific embodiments of the present invention will now be described with reference to the drawings, it should be understood that such embodiments are examples only and merely illustrative of but a small number of the many possible specific embodiments which can represent applications of the principles of the present invention. Various changes and modifications obvious to one skilled in the art to which the present invention pertains are deemed to be within the spirit, scope and contemplation of the present invention as further defined in the appended claims.

It should also be pointed out that though drawing illustrations and description are devoted to a helical three-lobe Roots supercharger in the present invention, the principle can be applied to other types of rotary supercharger with different numbers of lobes such as four-lobed, five-lobed or six lobed, etc. as long as both rotors have the same number of lobes. The principle can also be applied to either gas or liquid media, such as helical lobe or helical gear pumps that are variations of helical Roots superchargers for liquid and the later uses involute lobe shape to allow the lobes function as gears with rolling interfacial contact. In addition, helical lobe expanders are the above variations too except being used to generate shaft power from a media pressure drop.

As a brief introduction to the principle of the present invention, FIGS. 3 a to 3 d show again a complete cycle of Roots compression for a three-lobe Roots supercharger but with an addition of a shunt (parallel) pulsation trap of the present invention. In broad terms, pulsation traps are used to trap AND attenuate pulsations from compressed air or gas in order to reduce air borne pulsations discharged to atmosphere or downstream applications. Discharge dampener is one type of pulsation trap (traditional type) which is connected in series with the transfer (compression) chamber and through which both fluid flow and pulsation waves pass. The shunt pulsation trap is another type of pulsation trap but connected in parallel with the transfer (compression) chamber. As illustrated in FIGS. 3 a and 3 b, the phases of flow suction and trapping are still the same as those shown in FIGS. 2 a and 2 b. But during compression phase, instead of waiting until opening at the outlet as conventional Roots supercharger, the trapped flow cell is pre-opened to an injection port (or trap inlet) that is at least one lobe span away from the supercharger inlet port (For a three-lobe supercharger, it is 120 degrees, a four-lobe supercharger, 90 degrees). The injection port is branched off from the transfer chamber into the pulsation trap as a parallel chamber that is also communicating with the supercharger outlet through a feedback region (trap outlet). Between injection and feedback region and within pulsation trap, there is various pulsation dampening means or pulsation energy recovery means or both, to control pulsation energy before it travels downstream. As shown in FIG. 3 c or 3 c, a series of waves is generated as soon as the trapped air is opened to the trap inlet due to a pressure difference between the pulsation trap (relates to outlet pressure) and trapped air (relates to inlet pressure): The generated pressure waves or shockwaves travel to low pressure side compressing the air inside the cell, and at the same time, the simultaneously generated expansion waves on high pressure side, together with part of reflected shockwaves, are entering the pulsation trap, and therein are being stopped and attenuated. Because waves travel at a speed about 5-20 times faster than the rotor tip speed, the attenuation is well under way even before the lobe tip reaches the outlet, hence discharging a pulsation-treated air. If the shunt chamber (pulsation trap) energy dissipating volume and dampening resistance are specifically designed for achieving optimum attenuation, the pulsation-treated air can be almost pulse free. After the compression and pulsation attenuation phase, the lobes of two rotors will engage, meshing out the pulse-free compressed air to outlet and return to inlet suction position to start next cycle, as shown in FIG. 3 d.

The principal difference with the conventional Roots supercharger is in the compression and dampening phase: instead of waiting and delaying the compression and attenuation action until the lobe tip reaches the outlet by using a serially-connected dampener silencer, the shunt pulsation trap would start compression and induce pulsations into the trap as soon as the trap inlet is exposed to the trapped flow cell after it is sealed from the inlet. It then dampens the pulsations within the trap simultaneously as the cell flow is being compressed before reaching the outlet. In this process, the flow cell being compressed and pulsations being attenuated are happening in parallel with each other instead of in series as in the conventional Roots supercharger. Or in another word, compression and pulsation dampening are conducted at the same time (hence the name parallel or shunt), not one after the other (in series).

There are several advantages associated with the parallel pulsation trap compared with the traditional serially connected dampener silencer. First of all, pulsating wave attenuation is separated from the main cell flow so that an effective attenuation will not affect the main flow cell, resulting in both higher compression efficiency and attenuation efficiency. In a traditional serially connected silencer, both pulsating waves and fluid flow travel together through the dampening elements inside the silencer where a better attenuation always comes at a cost of higher static pressure drop. So a compromise is often made in order to reduce pressure loss by sacrificing the degree of pulsation dampening or use a very large volume silencer in a serial setup.

Secondly, the parallel pulsation trap attenuates pulsation much closer to the pulsation source than a serial one and is capable of using a more effective pulsation dampening means without affecting main flow efficiency. It can be built as an integral part and conforming shape of the supercharger casing with a much smaller size and footprint; hence less weight and cost. By replacing the traditional serially connected silencer with an integral paralleled pulsation trap, it will be light in weight and compact in size which also reduces noise radiation surfaces and is more suitable for mobile applications.

Moreover, the pulsation trap is so constructed that its inner casing is an integral part of the outer casing of the transfer chamber, and the outer casing are oversized surrounding the inner casing, resulting in a double-walled structure enclosing the noise source deeply inside the core with much less noise radiation surface area. The casings could be made from a casting that would be more wave absorptive, thicker and more rigid than a conventional sheet-metal silencer casing, hence less noise radiation.

With an integral pulsation trap, the supercharger outer casing would be structurally more rigid and resistant to stress or thermal related deformations. At the same time, the double-wall casing tends to have a more uniform temperature distribution inside the pulsation trap so that the traditional “banana shaped” casing distortion would be kept to minimum, thus reducing internal clearances and leakages, resulting in higher supercharger efficiency.

Referring to FIGS. 4 a-4 b, there are shown a typical arrangement of a preferred embodiment of a Roots supercharger 10 with a shunt pulsation trap apparatus 50. Typically, the Roots supercharger 10 has two parallel rotors 12 mounted on two rotor shafts respectively (not shown), where rotor shaft driven by an external rotational driving mechanism (not shown) and through a set of timing gears (not shown) drives the rotors 12 in synchronization without touching each other for propelling flow from an axial suction port 36 through a transfer chamber 37 to a discharge port 38 of the supercharger 10. The Roots supercharger 10 also has an inner casing 20 as an integral part of the transfer chamber 37, wherein the rotor shafts are mounted on an internal bearing support structure (not shown). The casing structure further includes an outer casing 28 with a space maintained between the inner casing 20 and the outer casing 28 forming the pulsation trap chamber 51.

As an important novel and unique feature of the present invention, a shunt pulsation trap apparatus 50 is conformingly surrounding the Roots supercharger 10 of the conventional design shown in FIG. 2 e, and its cross-section is illustrated in FIG. 3 e and FIG. 4 b. In the embodiment illustrated, the shunt pulsation trap apparatus 50 is further comprised of an injection port (trap inlet) 41 branching off from the transfer chamber 37 into the pulsation trap chamber 51 and a feedback region (trap outlet) 48 communicating with the supercharger outlet 38, therein housed pulsation dampening means 43 or pulsation energy recovery means (not shown). As lobe tip passes over the trap inlet 41 as shown for the right rotor in FIG. 3 e, a series of pressure waves are generated at trap inlet 41 going into the transfer chamber 37 inducing a feedback flow 53. Simultaneously a series of expansion waves are generated at trap inlet 41, but travelling in a direction opposite to the feedback flow, that is: from trap inlet 41, going through dampener 43 before reaching trap outlet 48 and supercharger outlet 38. In FIGS. 4 a-4 b, the large arrows show the direction of rotation and internal cell flow as propelled by the rotors 12 from the suction port 36 to the discharge port 38 of the supercharger 10, while feedback flow 53 as indicated by the small arrows goes from the feedback region (trap outlet) 48 through the dampener 43 into the pulsation trap chamber 51, then converging to the injection port (trap inlet) 41 and releasing into the transfer chamber 37 when lobe tip is opened up and becoming the compression chamber 39.

When a Roots supercharger 10 is equipped with the shunt pulsation trap apparatus 50 of the present invention, there exist both a reduction in pulsation discharged from Roots supercharger to supercharger downstream flow as well as an improvement in internal flow field (hence its adiabatic efficiency) so that it is compactly suitable for mobile applications, and efficiently suitable for applications typically reserved for conventional turbochargers or dry screw superchargers.

The theory of operation underlying the shunt pulsation trap apparatus 50 of the present invention is as follows. As illustrated in FIG. 3 a to FIG. 3 e and also refer to FIG. 4 a to 4 b, phases of flow suction and flow transfer are still the same as those shown in FIGS. 2 a and 2 b of a conventional Roots supercharger. But during compression phase, instead of waiting to be opened to supercharger outlet 38 as the conventional Roots supercharger does, the trapped flow cell inside the transfer chamber 37 is pre-opened to the injection port (or trap inlet) 41 that is at least one lobe span away from the inlet port 36 opening (For a three-lobe supercharger, it is 120 degrees; a four-lobe, 90 degrees). As shown in FIG. 3 e, a series of pressure waves or shock waves are produced due to a pressure difference between the pulsation trap chamber 51 (close to outlet pressure) and transfer chamber 37 (close to inlet pressure). The pressure waves traveling into the transfer chamber 37 (now becoming compression chamber 39) compress the trapped air inside, but at the same time, the accompanying expansion waves and a small portion of reflected pressure waves or shock waves enter the pulsation trap chamber 51, and therein are being stopped and attenuated by dampening means 43. Because waves travel at a speed about 5-20 times faster than the rotor 12 tip speed, the compression and attenuation are well under way even before the lobe tip reaches the supercharger outlet opening 38, hence discharging a pulsation-free or pulsation-reduced air. If pulsation trap volume and dampening resistance are specifically selected for achieving optimum attenuation, the pulsation reduction can be quite significant so that traditional externally connected outlet pulsation dampener or silencer is not needed anymore thus saving space and weight and suitable for mobile applications.

Moreover, the hot feedback flow 53 sandwiched between the cored and integrated inner casing 20 and outer casing 28 acts like a water jacket in a piston cylinder of an internal combustion engine, tending to equalize temperature difference between the cool inlet port 36 and hot outlet port 38. This would lead to less thermal distortion of the inner casing 20, which in turn would decrease the internal end clearance and tip clearance. In addition, by getting rid of the serially connected silencer, the associated discharge dampening losses are eliminated for the main cell flow At the trap inlet 41, the induced injection flow could be “choked” as pressure ratio across reaches 1.89, seriously limiting injection flow capacity and creating pressure losses. So using a flow nozzle 63 or de Laval nozzle 65, as shown in FIG. 4 b, would improve injection flow rate, injection time and flow efficiency compared to a traditional orifice shape so that supercharger overall adiabatic efficiency is greatly increased, hence suitable for applications typically reserved for turbochargers or dry screw superchargers.

FIG. 5 shows a typical arrangement of an alternative embodiment of the Roots supercharger 10 with a shunt pulsation trap apparatus 60. In this embodiment, a perforated plate 49 acting as a wave reflector and an additional dampener is added to the preferred embodiment as an additional means of the pulsation tarp 60.

FIG. 5 b and FIG. 5 c show wave reflector 49 is located either before or after feedback region (trap exit) 48 respectively. In theory, a wave reflector is a device that would reflect waves while let fluid go through without too much pressure losses. In this embodiment, the leftover pulsations either from the compression chamber 39 or coming out of pulsation trap outlet 48 or both could be further contained and prevented from traveling downstream causing vibrations and noises, thus capable of achieving more reductions in pulsation and noise but with additional cost of the perforated plate and some associated losses. With the feedback flow 53 going through the pulsation trap 51, the main discharge cell flow is unidirectional through the discharge wave reflector 49 as shown in FIG. 5 b without flow reversing losses and the associated dampening losses are greatly reduced too by using perforated holes with shape of either a flow nozzle 63 or de Laval nozzle 65 as shown in FIG. 6, thus improving discharge flow efficiency compared to a traditional Roots supercharger.

FIG. 7 shows a typical arrangement of yet another alternative embodiment of the Roots supercharger 10 with a shunt pulsation trap apparatus 70. In this embodiment, Helmholtz resonators 71 are used as an alternative pulsation eliminating means supplementing the pulsation trap 70. In theory, Helmholtz resonators could reduce specific undesirable frequency pulsations by tuning to the problem frequency thereby eliminating it. Since the Roots supercharger generates a specific single frequency pulsation when running at fixed speed and a Helmholtz resonator could be tuned to that specific frequency for elimination. In this embodiment, the pulsations generated at trap inlet 41 would be treated by Helmholtz resonator 71 located close to trap inlet 41 and in parallel with dampener 43. It could also be used alone or in multiple numbers or different sizes.

FIGS. 8-11 show some typical arrangements of yet another alternative embodiment of the Roots supercharger 10 with a shunt pulsation trap apparatus 80. In this embodiment, a diaphragm or a piston 81 is used as an alternative pulsation dampening and energy recovery (pumping) means for pulsation trap 80. FIG. 8 a shows an one-valve configuration, FIG. 8 b a two-valve, and FIG. 8 c a configuration with a dampener in place of the one-valve. In FIG. 8, the top view shows a charging (dampening) phase with only the trap inlet 41 and valve 82 open to the transfer chamber 37 while the trap outlet 48 and valve 83 are closed. In the same way, the bottom view shows a discharging (pumping) phase with the trap inlet 41 and valve 82 closed to the transfer chamber 37 while the trap outlet 48 and valve 83 open. The valves 82/83 used could be any types that are capable of being controlled and timed in the fashion as described above, and one example is given in FIG. 9 for a rotary valve. In operation, as an example shown in FIG. 8 b, a series of waves are generated as soon as the lobe tip pass over the pulsation trap inlet 41 during charging phase. The pressure waves would travel into the transfer chamber 37 while the accompanying expansion waves enter the pulsation trap chamber 51 in opposite direction. Because of the pressure difference between the pulsation trap chamber 51 (close to outlet pressure) and transfer chamber 37 (close to inlet pressure), the diaphragm 81 would be pulled towards the trap inlet 41 by the pressure difference hence absorbing the pulsation energy and storing it with the deformed diaphragm 81 (charged). At this time, the valve 83 located at the trap outlet 48 is closed, effectively scaling the waves within the pulsation trap chamber 51. As the rotor moves further and pressure difference is diminishing as shown in the bottom view of FIG. 8 b, the diaphragm 81 would be pulled away from the trap inlet 41 by the stored spring energy, resulting in a pumping action sucking air in from the now opened valve 83, building up the pressure again in the pulsation trap chamber 51 while trap inlet valve 82 is kept closed at this time. By alternatively open and close valves 82 and 83 in a synchronized way timed with the lobe and diaphragm positions, the pulsation energy could be effectively absorbed and re-used to keep the cycle going while the waves within the trap is kept contained and attenuated, resulting in a pulse-free air with minimal energy losses.

FIG. 10 is similar to FIG. 8 except using a piston instead of a diaphragm.

FIG. 11 shows a typical arrangement of yet another alternative embodiment of the Roots supercharger 10 with a shunt pulsation trap apparatus 80 a. In this embodiment, the diaphragm or a piston 81 is used as an alternative pulsation dampening and energy recovery (pumping) means for the pulsation trap 80 a. In the embodiment shown in FIG. 11 b, the difference with embodiment shown in FIG. 8 and FIG. 10 is that all or part of the pulsation energy stored is used to drive an external load 89, say a cooling fan.

FIG. 12 shows a typical arrangement of yet another alternative embodiment of the Roots supercharger 10 with a shunt pulsation trap apparatus 80 b. In this embodiment, a control valve 86 is used as pulsation containment means for pulsation trap 80 b, one on each side of discharge port 38. In addition, FIG. 12 shows a configuration with an optional dampener 43 between trap inlet 41 and control valve 86 located at trap outlet 48. The principle of the operation is taking advantages of the opposite travelling direction of wave and flow inside the pulsation trap 80 b. By using a directional controlled valve 86, it would only allow flow in while keeping the waves from going out of the trap in a timed fashion. In FIG. 12 b, the left rotor shows the wave containment phase with the trap inlet 41 open to the compression chamber 39 while the trap outlet 48 is closed by valve 86. In the same way, the right rotor shows a flow-in phase when the compression is finished and the trap outlet 48 is opened through valve 86. The valve 86 used could be any types that are capable of being flow controlled like a reed valve or timed with lobe rotation in a fashion as described above, and one example is given in FIG. 9 a for a rotary valve. In operation, as an example shown in FIGS. 12 a and 12 b, a series of waves are generated as soon as the lobe tip pass over the pulsation trap inlet 41 during isolation phase. The pressure waves would travel into the compression chamber 39 while the accompanying expansion waves enter the pulsation trap chamber 51 in opposite direction. At this time, the valve 86 located at the trap outlet 48 is closed, effectively sealing the waves within the pulsation trap chamber 51 where it is being dampened by an optional dampener 43 inside. As the rotor moves further and pressure difference is diminishing, the valve 86 at trap outlet 48 is opened allowing air in and building up pressure again in the pulsation trap chamber 51. By alternatively open and close valve 86 in a synchronized way timed with the lobe positions, the waves and pulsation energy could be effectively contained within the trap, resulting in a pulse-free air to the outlet.

It is apparent that there has been provided in accordance with the present invention a Roots supercharger with a shunt pulsation trap for effectively reducing the high pulsations caused by wave compression without increasing overall size of the supercharger. While the present invention has been described in context of the specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims. 

1. A Roots supercharger with a shunt pulsation trap apparatus, comprising: a. a housing structure having an inner casing with a flow suction port, a flow discharge port and a transfer chamber there-between, and at least one injection port located at least one lobe span away from said flow suction port communicating with said transfer chamber and at least one feedback region communicating with said flow discharge port, and an outer casing enclosing said inner casing; b. two parallel multi-helical-lobe rotors having same number of lobes and rotatably mounted on two parallel rotor shafts respectively inside said inner casing and interconnected through a set of timing gears to rotate in synchronization for propelling flow from said suction port to said discharge port: c. a shunt pulsation trap apparatus comprising said inner casing as an integral part of said transfer chamber, and said outer casing oversized surrounding said inner casing, therein housed various pulsation dampening means or pulsation energy recovery means or pulsation containment means, at least one trap inlet (said injection port) branching off from said transfer chamber into said pulsation trap and at least one trap outlet (said feedback region) communicating with said supercharger discharge port; d. whereby said Roots supercharger is capable of achieving high pulsation and NVH reduction at source and improving supercharger efficiency while being kept light in mass, compact in size and suitable for both mobile and stationary applications at the same time.
 2. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said multi-helical-lobe rotor is of twisted shape in its axial direction and having at least three or more lobes per rotor.
 3. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said injection port (trap inlet) is at least one lobe span away from said supercharger suction opening and has a converging cross-sectional shape or a converging-diverging cross-sectional (De Laval nozzle) shape in feedback flow direction.
 4. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means comprises at least one layer of perforated plate or acoustical absorption materials or other similar types for turning pulsation into heat.
 5. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means comprises at least one layer of perforated plate on which there is at least one synchronized valve that is closed and opened as said each lobe passes said trap inlet.
 6. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means comprises at least one Helmholtz resonator.
 7. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means comprises at least one Helmholtz resonator in parallel with at least one layer of perforated plate or acoustical absorption materials or other similar types for turning pulsation into heat.
 8. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means comprises at least one Helmholtz resonator in parallel with at least one synchronized valve that is closed and opened as each said lobe passes said trap inlet.
 9. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means and said energy recovery means comprise at least a diaphragm or a piston or other similar types in parallel with at least one layer of perforated plate or acoustical absorption materials or other similar types for partially turning pulsation into heat and partially absorbing pulsation energy and turning that energy into pumping air from said trap outlet through said perforated plate into said trap.
 10. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means and energy recovery means comprise at least a diaphragm or a piston or other similar types in parallel with an opening for absorbing pulsation energy and turning that energy into pumping air from said trap outlet through said opening into said trap.
 11. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means and energy recovery means comprise at least a diaphragm or a piston or other similar types synchronized with at least one valve for absorbing pulsation energy and turning that energy into pumping air from said trap outlet through said valve into said trap.
 12. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means and energy recovery means comprise at least a diaphragm or a piston or other similar types synchronized with at least one valve for absorbing pulsation energy and turning that energy into driving an externally connected load.
 13. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation dampening means and energy recovery means comprise at least a diaphragm or a piston or other similar types synchronized with at least two valves, one at trap inlet, the other at trap outlet, for absorbing pulsation energy and turning that energy into driving an externally connected load.
 14. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation trap further comprises at least one perforated plate located at said suction port or at least one perforated plate located at said discharge port or both either before or alternatively after said trap outlet
 15. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation containment means comprises at least one control valve located at said trap outlet.
 16. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 1, wherein said pulsation containment means comprises at least one layer of perforated plate or acoustical absorption materials or other similar types for turning pulsation into heat, in series with at least one control valve located at said trap outlet.
 17. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 15 or 16, wherein said control valve in said pulsation containment means is a one way valve, like a reed valve.
 18. The Roots supercharger with shunt pulsation trap apparatus as claimed in claim 15 or 16, wherein said control valve in said pulsation containment means is a rotary valve that is timed to close and open as each said lobe passes said trap inlet.
 19. The perforated plate as claimed in claim 14 has holes with a cross-sectional shape of either constant area or converging shape or a converging-diverging (De Laval nozzle) shape in flow direction. 